The present invention to a plant for producing high-pressure oxygen by air distillation.
The invention is applicable to the production of very large quantities of high-pressure oxygen, especially for supplying units producing synthetic hydrocarbons.
In this case, the pressures involved are absolute pressures.
Industrial units for producing synthetic hydrocarbons, called xe2x80x9cGTLxe2x80x9d (Gas-To-Liquid) units, commonly have a production capacity close to 50 000 barrels per day, which corresponds to an oxygen consumption of about 12 000 tonnes per day.
In order to produce such quantities of oxygen, it is necessary to provide several air distillation units in parallel, typically three or four units. Furthermore, in order to bring the oxygen to the high pressure needed for operation of the GTL unit, it is advantageous to bring the liquid oxygen produced by distillation to this high pressure by pumping, and to vaporize the liquid by heat exchange with a cycle fluid compressed to a high oxygen vaporization pressure, it being possible for this cycle fluid to be supercharged air or nitrogen. Thus, the use of gaseous oxygen compressors, always awkward, is avoided.
Consequently, each air distillation unit generally comprises two compression apparatus: on the one hand, a main air compressor, which brings all the treated atmospheric air to a medium distillation pressure (typically 5 to 8 bar for distillation in an apparatus comprising at least two distillation columns), and on the other hand, a cycle compressor, especially an air supercharger, which brings part of the compressed air stream to a high pressure enabling the pumped liquid oxygen to vaporize.
In the usual art, each main compressor is driven by a dedicated steam turbine, which may be a backpressure turbine, blowing off at a pressure greater than atmospheric pressure, or a condensation turbine, blowing off at a pressure less than atmospheric pressure and combined with a water condenser, cooled by water or by ambient air, and with a pump recycling the water to the steam production boiler. Similarly, each cycle compressor is driven by a dedicated steam turbine.
The result of this arrangement is a very high cost for the air compression means, especially when using condensation turbines, which are complex and expensive machines.
The aim of the invention is to reduce the investment associated with plants of the aforementioned type.
To this end, the subject of the invention is a plant for producing high-pressure oxygen by air distillation, characterized in that it comprises N (N greater than 1) air distillation units in parallel, each one of which comprises: a main air compressor adapted to compress the atmospheric air to be distilled; a steam turbine for driving the main compressor; means for precooling air; means for purifying air of water and of CO2; a main heat exchange line adapted to cool the compressed air down to a temperature allowing it to be distilled; an air distillation apparatus producing liquid oxygen and/or liquid nitrogen; at least one pump for pumping the liquid oxygen and/or the nitrogen to at least one high production pressure; and a cycle compressor adapted to compress a cycle fluid to a high pressure allowing the pumped liquid nitrogen and/or liquid oxygen to vaporize, characterized in that each cycle compressor is driven by an electric motor, and in that the plant comprises an (N+1)th steam turbine which drives an alternator, the latter supplying the electric motors with electric current.
The plant according to the invention may comprise one or more of the following characteristics:
the plant comprises N electric motors, each driving one of the cycle compressors;
the (N+1)th steam turbine is similar to each of the N other steam turbines;
the cycle compressor is an air supercharger adapted to supercharge part of the compressed air by the main air compressor;
each air distillation apparatus also produces gaseous nitrogen and each air distillation unit further comprises a gaseous nitrogen compressor adapted to compress the gaseous nitrogen to a high production pressure;
the electric current supply for the motors is supplemented by makeup coming from the grid;
the alternator produces an excess of electrical energy, which is exported to the grid; and
the plant comprises at least one unit for producing synthetic hydrocarbons, which unit is supplied with oxygen, and possibly with nitrogen, at their high production pressure.
One exemplary embodiment of the invention will now be described with regard to the appended drawing, the single FIGURE of which shows very schematically a plant according to the invention.